The world number one was left stunned as Cornet recovered from a first-set loss to beat the five-time champion and consign her to a third-round exit - her earliest elimination since 2005.
"Right now I don't really know what I did wrong," she told reporters.
"Usually I do. Usually I know I did this, that or that. I have a few ideas, but this will be a really good one for me to assess and figure out what I can do, to do better next time.
"That's happened to me a few times, but it's fun: it kind of gives you a mission to work on, gives you goals to work towards to see what you can do better."
Cornet faces Eugenie Bouchard in the next round.